1. Field of the Invention
This invention relates to the processing of device wafers where temperature must be carefully controlled during processing and, more particularly, to the utilization of non-contaminating, inert, polymer films to provide for thermal conduction from device wafers to a heat sink during processing.
2. Description of Prior Art
The field of semiconductor device fabrication includes several fabrication steps where substantial heat is generated. These steps include, for example, sputter deposition or etching, ion beam etching, vacuum evaporation, plasma etching, chemical vapor deposition, ion implantation and the like.
When fabricating semiconductor devices, the temperature must often be maintained at or close to a certain value. During etching by the techniques mentioned above, power inputs in the order of 0.1-1 watt/cm.sup.2 are commonly encountered. If photoresist etch masks are used, the temperature must be held below some maximum value to prevent loss of integrity of the photoresist pattern.
Likewise, ion beam etching, sometimes called ion milling, is used in place of wet chemical etching to define high resolution circuit patterns. During this process, high energy ions bombard the surface of the wafer on which a protective pattern has been formed in photoresist or electron resist, thus removing material from the unprotected portion of the device wafer by a momentum transfer process known as physical sputtering. The power density of this ion bombardment is typically in the neighborhood of one watt per cm.sup.2, about 95 percent of which is converted to heat. Thus, the device wafer will rapidly heat up, destroying the resist pattern.
The methods which are currently employed to avoid the harmful effects of heating during ion beam etching are reduction of etch rate, use of heat impervious masking patterns instead of photoresist or electron-sensitive resist, and the use of liquid metals (such as gallium or gallium-indium alloys) or a grease or wax between the wafer and the heat sink as a medium to assure good thermal contact. Each of those methods has significant drawbacks. Reduction of milling rate to lower the rate of heating is generally unacceptable since material removal rates must be limited to typically 10-50 A/min. for resist protection. This rate results in extremely long ion etching cycle times for typical thin films several thousand angstroms thick. Use of a metal etch mask pattern instead of a resist pattern merely adds processing steps and shifts the problem to that of producing the metal pattern on the wafer and removing it after ion beam etching. Photoresist is used to form the metal mask pattern, and if it is a high resolution pattern, a technique such as ion etching must be used to etch the mask, so nothing is gained. The use of vacuum grease or gallium to provide thermal contact between wafer and heat sink is currently the most popular method of limiting the temperature excursion of a wafer being ion etched. However, the process of applying and removing the contacting medium is messy and labor consuming and inevitably has a negative impact on yield. The additional labor required to implement these cooling techniques is generally an equal tradeoff at best for the time gained by somewhat greater processing rates.
Thus, there is a need to utilize processes and tooling which are not complex for improving the temporary thermal conduction between the device wafer and the mounting surface without introducing additional steps or contamination and yet assuring no physical damage of the device wafer during dismounting or damage to the resist masks during the fabrication processes.